TABLE OF CONTENTS This chapter precedes the chapter on centrifugal compressors for a sound reason. Damage to high-speed centrifugal compressors is most often associated with slugs of liquid entering the compressor from an upstream knockout drum. The other major use of knockout drums is to separate droplets of liquid from gas streams to be used for fuel or further processing. Separating droplets of water from liquid hydrocarbon is also discussed in this chapter.
The majority of the process vessels you see in your plant are gravity, vapor-liquid separators. Their main purpose is to settle out droplets of entrained liquid from the upflowing gas. Factors that affect the settling rate of these droplets are
Droplet size. Big droplets settle faster than little droplets. Contrary to that nonsense we were taught in school, cannon balls do drop faster then BBs if one accounts for the resistance of air (BBs are small spheres of lead shot used in air guns).
Density of vapor. The less dense the vapor, the faster the droplet settling rate.
Density of liquid. The more dense the liquid droplets, the faster the droplet settling rate.
Velocity of vapor. The slower the vertical velocity of the vapor, the faster the settling rate.
Viscosity of the vapor. The lower the vapor viscosity, the faster the settling rate.
All these ideas have been put into one equation, called Stokes law. Nothing against Sir Frederick Stokes, but vapor viscosities are almost always so small that they do not...
